1. Field of the Invention
This invention relates generally to lubricating oil compositions and specifically to such an oil composition which has high oxidative stability at elevated temperatures.
2. Prior Art
It is widely accepted that a 2,6-di-tert-butyl-p-cresol, a compound known as DBPC, is highly effective as an oxidation inhibitor for lubricating oils such as typically turbine oil. However, DBPC per se of a relatively low molecular weight is susceptible to a reduction in its concentration by evaporation upon exposure to elevated temperature over prolonged length of time, resulting in a loss of oxidative stability.
Japanese Laid-Open Patent Publication No. 60-156644 discloses the use of sterically hindered hydroxyphenylcarboxylic acid ester as a stabilizing agent for synthetic organic polymers, animal and vegetable oils, hydrocarbons, lubricants and the like. A specific example of this stabilizing agent is disclosed to be (3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid ester. While this compound is much more resistant to diminution by evaporation compared to 2,6-di-tert-butyl-p-cresol, it is less effective in terms of oxidation-inhibition.
A demand is acknowledgeable for high oxidation resistant lubricants capable of use in gas turbines, compressors, hydraulically actuated machines and the like operative at extremely high temperatures that have become increasingly compact in size and longer in service life.
As a result of extensive research and development directed to the discovery of oxidation-resistant lubricant compositions comparable or even superior to the conventional 2,6-di-tert-butyl-p-cresol, the present inventors have found that a surprisingly effective lubricant composition can be provided by the use of a predetermined amount of a 3-methyl-5-tert-butyl-4-hydroxyphenyl substituted fatty acid ester as an additive to a highly refined base oil such as hydrocracked oil or certain synthetic oils, as disclosed in Japanese Patent Application No. 6-330532.
It has later been found however that the additive of 3-methyl-5-tert-butyl-4-hydroxyphenyl substituted fatty acid ester as used in a base oil sufficiently or not sufficiently refined fails to exhibit particularly effective oxidation inhibiting capability over phenol-based oxidation inhibitors including 2,6-di-tert-butyl-p-cresol when applied under relatively mild temperature, oxygen pressure and prolonged oxidative environment.
It has now been found that the use of an auxiliary component which is one member of the group consisting of a specific nitrogen-containing compound, a specific sulfur and/or phosphorus-containing compound and a specific phenolic compound as combined in a predetermined proportion with 3-methyl-5-tert-butyl-4-hydroxyphenyl substituted fatty acid ester is highly conducive to the availability of an improved lubricating oil composition which is capable of sustained oxidative stability under severe operating conditions over extended periods of time.
It has also been found that the use of an auxiliary component consisting essentially of the nitrogen-containing compound and the sulfur and/or phosphorus compound leads to further improvement in oxidative stability.
According to the invention, there is provided a lubricating oil which comprises
(A) a base oil, PA1 (B) a 3-methyl-5-tert-butyl-4-hydroxyphenyl substituted fatty acid ester in an amount of 0.1-5 percent by mass represented by the formula ##STR1## wherein R.sup.1 is an alkylene group of 1-6 carbon atoms and R.sup.2 is an alkyl or alkylene group of 1-24 carbon atoms, and PA1 (C) an auxiliary component, said auxiliary component being one member of the group of: PA1 (A-1) a mineral oil having a kinematic viscosity of 5-200 mm.sup.2 /s at 40.degree. C. and an aromatics content of less than 15 percent by mass. PA1 (A-2) an olefinic polymer of 2-16 carbon atoms or its hydrogenated product having a number-average molecular weight of 250-4,000. PA1 (A-3) an alkylbenzene having 1-4 alkyl groups of 1-40 carbon atoms and a total carbon number of said alkyl groups in the range of 6-40.
(a) a nitrogen-containing compound in an amount of 0.001-1 percent by mass, said nitrogen-containing compound being selected from the group consisting of thiadiazole, benzotriazole and/or the derivative thereof and benzothiazole; PA2 (b) a sulfur and/or phosphorus-containing compound in an amount of 0.1-5.0 percent by mass, said sulfur and/or phosphorus-containing compound being selected from the group consisting of sulfides, phosphides, dithiophosphates and dithiocarbamates; or PA2 (c) a phenolic compound in an amount of 0.1-5.0 percent by mass, said phenolic compound being selected from the group consisting of a compound (c-1) of the formula ##STR2## wherein R.sup.36 is an alkyl group of 1-4 carbon atoms, a group of the formula ##STR3## wherein R.sup.37 is an alkylene group of 1-6 carbon atoms and R.sup.38 is an alkylene or an alkyl group of 1-24 carbon atoms, or a group of the formula ##STR4## wherein R.sup.39 is an alkylene group of 1-6 carbon atoms, and a compound (c-2) of the formula ##STR5## wherein R.sup.40 and R.sup.41 each are an alkylene group of 1-6 carbon atoms and X is an alkylene group of 1-18 carbon atoms or a group of the formula EQU --R.sup.42 --S--R.sup.43 -- (iii) PA4 wherein R.sup.42 and R.sup.43 each are an alkylene group of 1-6 carbon atoms, PA2 wherein R.sup.42 and R.sup.43 each are an alkylene group of 1-6 carbon atoms.
the foregoing percentages by mass being based on total composition.
According to a preferred embodiment of the invention, the auxiliary compound consists essentially of the nitrogen-containing compound and the sulfur and/or phosphorus compound.